Many modern electronic devices, such as mobile wireless communication devices, include integrated circuits having various nonlinear components, such as transistors and amplifiers. For example, the trend towards increasing energy efficiency, e.g., to extend battery life of mobile wireless communication devices, requires the transistors to operate under increasing nonlinear conditions. Also, wider bandwidths of modern signal formats, such as long term evolution (LTE), WiMAX and wideband code division multiple access (WCDMA), are stimulating the transistors with complex modulation formats, including more complicated signals with higher peak-to-average ratios. Further, transistors fabricated in new semiconductor materials, such as gallium nitride (GaN), and other compound semiconductor materials, such as gallium arsenide (GaAs), may exhibit complicated nonlinear dynamical effects in response to the complex modulation signals. Consequently, test instruments, such as Nonlinear Vector Network Analyzers (NVNAs) and Large-Signal Network Analyzers (LSNAs), need the ability to accurately measure and characterize these nonlinear components and/or characteristics.
For example, when measuring intermodulation distortion (IMD) and memory effects of a device under test (DUT), such as an amplifier, an NVNA stimulates the DUT with a multi-tone stimulus signal and receives a corresponding multi-tone response signal, in which two or more tones are localized around a center frequency of the stimulus signal. If the tones in the multi-tone stimulus and response signals are too closely spaced to one another (e.g., less than 1 MHz apart), then a conventional phase reference signal, consisting of multiple reference tones at predetermined intervals, may not be able to provide enough signal power at each of the reference tones for the NVNA to obtain a good measurement. That is, in order to measure phase of the multi-tone response signal, the reference tones of the phase reference signal must be at least as closely spaced as the tones in the multi-tone response signal. However, the smaller the interval between the reference tones, the lower the power of each reference tone, until the reference tones of the phase reference signal are lost in the noise floor, resulting in noisy measurements. For this reason, conventional test instruments are not able to use multi-tone stimulus signals with very narrow spacing between the tones, e.g., less than 1 MHz, as a practical matter.
Generally, a conventional phase reference signal is generated by a phase reference signal generator, which is essentially a pulse generator clocked by a fixed reference signal. When viewed in the time domain, the phase reference signal is a pulse train. When viewed in the frequency domain, the phase reference signal is a broadband comb of reference tones, referred to as a grid, which are spaced apart at intervals equal to the reference signal. FIG. 1 is a graph showing power distribution among the reference tones of a conventional phase reference signal at different tone spacing intervals in the frequency domain. As shown in FIG. 1, grid 101 is a high frequency (1 GHz) grid, in which the spacing interval between adjacent reference tones (e.g., integer multiples of 1 GHz) is relatively large and the power is about −40 dBm. In comparison, grid 102 is a low frequency (100 MHz) grid, in which the spacing interval between adjacent reference tones (e.g., integer multiples of 100 MHz) is relatively small. The power, which is distributed over a larger number of reference tones in grid 102, is about −60 dBm. Accordingly, the power associated with each reference tone in gird 102 is about 20 dB lower than the power associated with each reference tone in gird 101, resulting in noisy measurements at the lower frequency.